An internal combustion engine typically has an exhaust gas purification catalyst provided in an exhaust passage to purify exhaust gas. Among such catalysts, a catalyst having a capability of storing oxygen, such as a three way catalyst, is capable of removing NOx and removing HC and CO by oxidation with release of oxygen stored in it. The exhaust gas purification catalyst having a capability of storing oxygen as described above is intended to remove NOx, HC, and CO at the same time by making the air-fuel ratio of the exhaust gas flowing into the catalyst richer than the theoretical air fuel ratio and leaner than the theoretical air fuel ratio alternately.
As an exhaust gas purification catalyst deteriorates, its exhaust gas purifying ability deteriorates. Therefore, as deterioration of the catalyst progresses to some extent, the catalyst needs maintenance such as replacement. For exhaust gas purification catalysts having a capability of storing oxygen like three-way catalysts, a technique of determining deterioration of a catalyst by detecting a decrease in its oxygen storage capacity (see, for example, patent literature 1). In this technique, the air-fuel ratio of the exhaust gas flowing into the exhaust gas purification catalyst is shifted from lean to rich and from rich to lean alternately on the basis of measurement values of an air-fuel ratio sensor provided downstream of the oxidation purification catalyst. The oxygen storage capacity of the exhaust gas purification catalyst is calculated based on the difference in the exhaust gas air-fuel ratio and the flow rate of the exhaust gas flowing through the exhaust gas purification catalyst in this process, and deterioration of the catalyst is determined based on the calculated oxygen storage capacity.